Our studies have concerned regulation of gene expression during normal and abnormal differentiation processes. Tunicamycin was found to suppress placental alkaline phosphatase (PAP) biosynthesis in addition to inhibiting protein glycosylation. The degradation rate of the unglycosylated PAP monomer was unchanged. Tunicamycin suppressed PAP mRNA activity leading to the observed decrease in biosynthesis. Genes encoding PAP were cloned. Efficient isolation of these genes was accomplished by probing a phage Lambdagt ll recombinant DNA expression library with polyvalent antibodies directed against highly purified PAP. Alpha-Fetoprotein (AFP) synthesized by the temperature-sensitive (ts) fetal liver cells exhibiting a transformed phenotype was a glycoprotein of 65,000 daltons. The unglycosylated form of the 65,000-dalton AFP was a polypeptide of 48,000 daltons. Whereas the unglycosylated form of mature AFP from normal fetal liver was a polypeptide of 66,000 daltons. These two AFP variants were arised by translation of two mRNAs of l6S and 20S which were produced from a single AFP gene. The l6S AFP RNA differed from the 20S RNA at the 5' end. The SV40 tsA mutant-transformed adult hepatocytes are ts for growth and differentiation. They synthesized greatly increased levels of albumin and transferrin and had greatly increased levels of albumin and transferrin mRNAs at 40 degrees C (normal phenotype). These adult hepatocytes expressed 2 additional liver-specific functions, tyrosine aminotransferase (TAT) and fibrinogen. Such expression was temperature-sensitive and glucocorticoid dependent. Regulation of the expression of these liver-specific functions appears related; the studies should aid in the understanding of the differentiation processes. HepG2 hepatoma cells produced two species of transferrin with apparent molecular weights of 74,500 daltons, predominantly extracellar, and of 72,000 daltons, predominantly intracellar. Human transferrin has been shown to be a single polypeptide of 73,200-79,500 daltons. Production of a second transferrin variant by HepG2 cells may indicate an altered mechanism in transferrin gene regulation.